What characterises individual indoor pollutants? What are the sources of pollutants? How do these chemicals affect humans and the environment? And how does the eco-INSTITUT judge the substances? These and other questions are answered in our “pollutant profile” section.
Isocyanates are organic compounds that serve primarily as starting substances for the production of polyurethane-based plastics. They are the result of the reaction of amines and the gas phosgene. A distinction is made between mono-, di- and poly-isocyanates according to the number of isocyanate groups present in an isocyanate molecule. Isocyanates are highly reactive and combine quickly with e.g. water, alcohols or amines to form long-chain compounds.
Diisocyanates are the technically most important group of isocyanates: By reacting with alcohols or phenols, the versatile polyurethanes (PUR) are generated, which are mainly used as foams, lacquers, adhesives, coatings or in assembly foams.
Diisocyanates are for instance:
MDI – diphenylmethane-4,4′-diisocyanate
HDI – 1,6-hexamethylene diisocyanate
TDI – toluene-2,4-diisocyanate
Isocyanates irritate the eyes, skin and mucous membranes and have a strong allergenic effect; some are suspected of being carcinogenic.
Monoisocyanates such as methyl isocyanate are used, for example, in the production of pesticides.
Diisocyanates are technically important: they are used in the production of polyurethanes (PUR) and thus as raw materials for adhesives, paints, coatings, assembly foams and insulation boards, among other things. PUR foams are also used to fabricate mattresses. In total, more than 90 % of isocyanates are used for the production of polyurethanes.
Isocyanates can enter the body through the respiratory tract or the skin.
Employees in factories where isocyanates are produced or processed are directly at risk. Also at risk are the workers who process certain products containing isocyanates, which react to form polyurethanes only during use (e.g. one- or two-component PUR adhesives, varnishes or assembly foams). This may cause high isocyanate emissions. The so-called monomeric isocyanates – i.e. isocyanates that have not been polymerised and are therefore still reactive molecules (see excursus*) – are of great relevance to health; although monomers are usually only present in small quantities in products containing isocyanates, they are very volatile and can thus be easily inhaled.
Once isocyanates have been converted (i.e. all isocyanate groups have been used up and chemically transformed) – e.g. in the case of cured PUR paints or wood-based panels containing isocyanates – they generally no longer pose a health hazard. However, isocyanates can be released unintentionally: e.g. during the thermal decomposition of plastics and during the storage or mechanical processing of incompletely converted PUR products.
Effects on humans and the environment
Isocyanates have a strong irritating effect on eyes, skin and mucous membranes – even in low concentrations. In higher doses, they cause, among other things, severe coughing and shortness of breath. In addition, isocyanates are highly allergenic: they cause sensitisation of the skin and respiratory tract (this is also referred to as isocyanate asthma). The allergising effect can already occur at concentrations far below the workplace limits.
Some isocyanates are considered to be possibly carcinogenic.
The gas phosgene, which is needed for the production of isocyanates, is highly toxic and hazardous to water (phosgene was used as a chemical warfare agent in World War I).
In 1984, isocyanates – or more precisely methyl isocyanate – were the cause of one of the biggest chemical disasters in history: In Bhopal, India, a tank containing the chemical exploded in a pesticide factory, causing tens of thousands of deaths and hundreds of thousands of injuries.
* Excursus: Monomeric isocyanates are single molecules with reactive isocyanate groups – as monomers they are a building block for the construction of larger molecules, the polymers. Oligomers are medium-sized molecules that are composed of several monomeric isocyanates or can also represent short chains of the polymer. Short chains of the polymer with reactive isocyanate groups are also called prepolymers. These are prepolymerised raw materials which – in the case of polyurethane – are produced by the reaction of isocyanates with polyols (special alcohols). In products containing isocyanates, isocyanates are usually present as prepolymers or as oligomers of several monomeric isocyanates, and as monomers only in small proportions. In contrast to monomers, oligomers and prepolymers are less volatile.
Various regulations such as the CLP Regulation, TRGS 905, IARC list and MAK list classify a number of isocyanates as potentially carcinogenic, including the following substances:
Prohibitions and restrictions of use
Occupational exposure limit values (OELs) are specified for monomeric diisocyanates.
In addition, monomeric diisocyanates will in future be subject to restrictions on industrial and commercial use and on placing on the market: According to REACH -regulation, diisocyanates may no longer be used industrially or commercially in other substances or mixtures throughout the EU from 24.8.2023 – unless the concentration of diisocyanates is below 0.1% by weight or the users are appropriately trained for safe handling. Suppliers who place such products on the market must ensure that their customers are aware of the requirements and that there is a reference to the future training obligation on the product packaging as from 24.2.2022.
The final consumer is also to be protected: According to Annex XVII REACH -regulation*, restrictions on the use of products containing MDI have applied since 27.12.2010: If a mixture contains ≥ 0.1% by weight of MDI, it may not be sold to the general public – with one exception: protective gloves are included in the packaging. The industry has wrested this compromise from the EU.
* Regulation (EU) 2020/1149 of 3 August 2020 Annex XVII of Regulation (EC) No 1907/2006 of the European Parliament and of the Council on the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) as regards diisocyanates.
In principle, certain products in which isocyanates are used can be awarded the eco-INSTITUT-Label. These include, for example, wood-based panels or floor coverings with isocyanate-containing materials (e.g. adhesives), products with a PUR coating or mattresses made of PUR foam. In these cases, the isocyanates have already been converted to a polymer and no longer pose a risk to the consumer.
For safety reasons, however, these products are tested for the monomeric diisocyanates HDI, TDI and MDI as part of the emission measurement. For HDI, 2,4-TDI and 2,6-TDI there is a requirement of < 1 µg/m³, for MDI the requirement is < 2 µg/m³.
It is ultimately a balancing act between the potential environmental and health hazards of isocyanate production and processing and their benefits in use – compared to other products. For example, wood-based panels containing isocyanates have the advantage over products glued with glue containing formaldehyde in that overall emissions are lower.
When deciding to use isocyanate-containing components, eco-INSTITUT recommends that manufacturers always check whether technically comparable solutions exist and whether the environmental and health hazards of the alternatives are lower.
Liquid products (e.g. adhesives or liquid sealants) that contain isocyanate-containing components cannot be certified in principle. Even if oligomers or prepolymers are used, residual hazards may still emanate from the products. Such products are labelled on the packaging with the so-called EUH phrase EUH204 “Contains isocyanates. May cause allergic reactions.” ( the EUH phrases applicable only in the EU supplement the hazard statements for products defined in the CLP Regulation).
Bundesanstalt für Arbeitsmedizin und Arbeitsschutz (BAuA) (2022): Beschränkung von Diisocyanaten unter REACH: Was industrielle und gewerbliche Verwender und Lieferanten beachten müssen. Helpdesk kompakt: REACH. (https://www.baua.de/DE/Angebote/Publikationen/Praxis/Diisocyanate.pdf?__blob=publicationFile&v=2)
Messung und Beurteilung von Isocyanaten unter Beachtung der TRGS 430: Gefahrstoffe – Reinhaltung der Luft (2013) Nr. 5 (https://www.dguv.de/medien/ifa/de/pub/grl/pdf/2013_122.pdf)
Richtwerte für die Innenraumluft – Diisocyanate: Bundesgesundheitsblatt – Gesundheitsforschung – Gesundheitsschutz 43 (2000) (https://www.umweltbundesamt.de/sites/default/files/medien/pdfs/Diisocyanate.pdf)
Rühl R., Rouw A. (2018): Isocyanate (Teil 1 von 3). sicher ist sicher 11.18: 482 – 487.
Graphic: Karin Roth